x86: Annotate idtentry_df()

Without CONFIG_X86_ESPFIX64 exc_double_fault() is noreturn and objtool
is clever enough to figure that out.

vmlinux.o: warning: objtool: asm_exc_double_fault()+0x22: unreachable instruction

0000000000001260 <asm_exc_double_fault>:
1260:       f3 0f 1e fa             endbr64
1264:       90                      nop
1265:       90                      nop
1266:       90                      nop
1267:       e8 84 03 00 00          call   15f0 <paranoid_entry>
126c:       48 89 e7                mov    %rsp,%rdi
126f:       48 8b 74 24 78          mov    0x78(%rsp),%rsi
1274:       48 c7 44 24 78 ff ff ff ff      movq   $0xffffffffffffffff,0x78(%rsp)
127d:       e8 00 00 00 00          call   1282 <asm_exc_double_fault+0x22> 127e: R_X86_64_PLT32    exc_double_fault-0x4
1282:       e9 09 04 00 00          jmp    1690 <paranoid_exit>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/Yi9gOW9f1GGwwUD6@hirez.programming.kicks-ass.net

arch/x86/entry/entry_64.S

@@ -551,6 +551,9 @@ SYM_CODE_START(\asmsym)
 	movq	$-1, ORIG_RAX(%rsp)	/* no syscall to restart */
 	call	\cfunc
 
+	/* For some configurations \cfunc ends up being a noreturn. */
+	REACHABLE
+
 	jmp	paranoid_exit
 
 _ASM_NOKPROBE(\asmsym)